Defect Inspection Method

ABSTRACT

A method for inspecting a defect of a surface of a sample includes irradiating a laser beam on the sample surface a plurality of times so that at least part of an illumination field of the laser beam on the sample surface illuminates a first area of the sample surface each of the plurality of times, detecting a plurality of scattered light rays from the first area caused by the plurality of times of irradiations, correcting errors of detection timings for the plurality of detected scattered light rays, correcting at least one of adding and averaging the plurality of scattered light rays, determining a defect on the sample surface based on a calculation result in accordance with the at least one of the adding and averaging.

CROSS REFERENCE TO RELATED APPLICATION

The present application is related to U.S. application Ser. No.12/109,548, filed Apr. 25, 2008 by some of the inventors herein.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese Patent applicationNo. 2008-022255 filed on Feb. 1, 2008, the contents of which is herebyincorporated by reference herein.

BACKGROUND

The present invention relates to a defect inspection method forinspecting a minute defect existing on a sample surface and a defectinspection apparatus therefor.

In production lines of semiconductor substrates, thin film substrates,etc., in order to maintain and improve the yield of a product,inspection of a defect that may exist on the surface of semiconductorsubstrates, thin film substrates, etc. is performed. As conventionaltechnologies, JP-A-Hei9(1997)-304289 (patent document 1) andJP-A-2000-162141 (patent document 2) are known. In order to detect aminute defect, a laser beam focused to a few tens μm is irradiatedthereon and scattered light from the defect is converged and detected.

In connection with rapid miniaturization of LSI wiring in recent years,the size of a defect that should be detected is approaching a detectionlimit of optical inspection. According to the semiconductor load map,mass production of LSI of a 36-nm node is going to be started in 2011,and a capability of detecting the defect having a size of about a halfof DRAM ½ pitch is considered required. It is known that a magnitude Iof scattered light occurring when the defect is illuminated by a laserhas a relation of I∝d⁶, where d denotes a particle size of the defect.That is, when the defect size becomes small, the scattered lightoccurring thereby will decrease rapidly. Although, as methods ofincreasing the scattered light occurring, there exist wavelengthshortening of illumination wavelength, power increasing of a laser,reduction of a laser illumination spot, etc., any of these methods comeswith a possibility of giving damage to the sample due to an increase oftemperature of an irradiated portion.

SUMMARY

An object of the present invention is to provide a defect inspectionmethod and apparatus capable of inspecting a minute defect existing on asample surface with high sensitivity while suppressing damage to thesample.

As a method of increasing the scattered light that is to be detectedwhile suppressing temperature rise of an irradiated portion, consideredis a method whereby approximately the same area is irradiated aplurality of times and a plurality of detected scattered light rays areadded. However a plurality of scattered light rays from approximatelythe same area that are detected actually by this method may have errorsof detection timings that arise from position shifts and angle shifts ofan illumination apparatus, inclination of the sensor, etc. In such acase, it is considered that if the plurality of detected scattered lightrays are added as they are, the scattered light rays cancel to oneanother, and consequently defect detection sensitivity falls. To solvethis problem, the present application proposes a method for correctingdetection timing errors that the plurality of scattered light rays have.

Representative aspects among aspects of the invention disclosed by thepresent invention will be explained briefly as follows:

(1) One of the aspects of the present invention is a defect inspectionmethod of the sample surface, including: an irradiation step ofirradiating a laser beam on a first area of the sample surface aplurality of times; a detection step of detecting a plurality ofscattered light rays from the first area caused by the plurality oftimes of irradiation; a correction step of correcting the errors of thedetection timings that the plurality of scattered light rays detected inthe detection step have; a step of adding or averaging the plurality ofscattered light rays corrected in the correction step; and a defectdetermination step of determining a defect on the sample surface basedon a calculation result by the step of adding or averaging.(2) An other of the aspects is the defect inspection method described in(1), wherein in the correction step, the correction is performed using adetection timing error value that is decided using a reference waferhaving a reference point that is a defect whose position is known.

These features and advantages of the invention will be apparent from thefollowing more particular description of preferred embodiments of theinvention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a first embodiment of a defect inspection apparatus accordingto the present invention.

FIG. 2 is a detection optical system in the first embodiment of thedefect inspection apparatus according to the present invention.

FIG. 3 is a top view of the first embodiment of the defect inspectionapparatus according to the present invention.

FIG. 4 is an explanatory diagram about a method for illuminatingapproximately the same area of a sample a plurality of times.

FIG. 5 is a defect detection processing flowchart showing a defectinspection method according to the present invention.

FIGS. 6A-6C are explanatory diagrams about operations such as additionand errors of detection timings of a plurality of scattered light rays.

FIG. 7 is a flowchart about a calibration method.

FIG. 8 is an explanatory diagram about a method for designating a stagerotation center.

FIG. 9 is an explanatory diagram about a method for deciding anirradiation position and its direction.

FIG. 10 is an explanatory diagram about a method for monitoring anillumination position.

FIGS. 11A-11B are explanatory diagrams about a method for correcting aninclination of illumination on a wafer.

FIG. 12 is an explanatory diagram about a sensor whose sensor lightreceiving part is high, and the illumination position and inclination.

FIG. 13 is an explanatory diagram about a sensor whose sensor lightreceiving part is low, and the illumination position and inclination.

FIG. 14 is an explanatory diagram about the errors of the detectiontimings of the plurality of scattered light rays.

FIGS. 15A-15C are explanatory diagrams of a correction method of theerrors of the detection timings.

FIG. 16 is an explanatory diagram of a switch method of storage memory.

FIG. 17 is an explanatory diagram of the switch method of the storagememory in a wafer outermost peripheral part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One example of embodiments of a defect inspection apparatus according tothe present invention will be explained using FIG. 1. The defectinspection apparatus shown by FIG. 1 is configured by having anillumination optical system 101, a detection optical system 102, a waferstage 103, a circuit 110, and a signal processing section 111. Theillumination optical system 101 is constructed using a laser lightsource 2, a beam expander 3, a homogenizer 4, mirrors 5, 6, and acollective lens 7. A laser beam 100 emitted from the laser light source2 is adjusted in beam diameter by the beam expander 3 so that it mayhave a desired size, is converted by the homogenizer 4 so as to have auniform illuminance distribution, and is made to conduct linearillumination in an area under inspection of a wafer 1 by the collectivelens 7. Here, as the laser light source 2, what is necessary is just touse a laser light source oscillating an ultraviolet or vacuumultraviolet laser beam. The beam expander 3 is an anamorphic opticalsystem, and is constructed using a plurality of prisms. The beamexpander 3 changes a beam diameter only in one direction in a planeperpendicular to the optical axis and conducts linear illumination onthe sample using the collective lens. The irradiation may be conductedusing a cylindrical lens, not with a combination of the collective lens7 and the expander 3. When a single body of cylindrical lens is used, itis effective in a point that slimming of the optical system can beattained. Moreover, although the homogenizer 4 is used in order to makeillumination intensity uniform, an illuminance distribution may be madeuniform by using a diffraction optical element or fly eye lens, forexample. Further, the illumination may be performed without using thehomogenizer 4. Omission of the homogenizer makes it possible to suppressattenuation of the laser beam intensity and to conduct illumination atan intense illuminance.

The detection optical system 102 is constructed by having an imageformation system 8 and a photodiode array 9. The detection opticalsystem 102 will be explained in detail using FIG. 2. The detectionoptical system 102 is constructed using a collective lens 21, an imageintensifier 22, an image formation lens 23, and the photodiode array 9.Light rays scattered from an illumination field 20 are converged by thecollective lens 21, and the scattered light rays are amplified by theimage intensifier 22 and imaged onto the photodiode array 9 via theimage formation lens 23. Here, although the image intensifier 22 is usedin order to amplify the scattered light so that weak scattered light maybecome detectable, a sensor that has a high amplification ratio itself,for example, EM-CCD, a multi-anode PMT, etc. may be used, even withoutusing the image intensifier. If any of these sensors is used, it will beeffective in a point that slimming of the apparatus can be attained. Inaddition, the photodiode array 9 is used in order to receive thescattered light and perform photoelectric conversion on it. A TV camera,a CCD linear sensor, a TDI, the multi-anode PMT, a two-dimensionalsensor, or the like may be used instead of it. It becomes possible toinspect a large area at once, for example, by using a two-dimensionalsensor.

This photodiode array 9 generates an electric signal in proportional tothe amount of received light. The electric signal generated by thephotodiode array 9 undergoes necessary amplification, noise processing,and analog-to-digital conversion in the analog circuit 110. A pluralityof optical signals that are of scattered light rays from approximatelythe same area are added and processed by defect determination in thesignal processing section 111, and results lead to display of the defectmap in a map output section 113 via a CPU 112. The wafer stage 103 isconstructed with a chuck for holding the wafer 1 (not shown), a rotationstage 10 for rotating the wafer 1, and a translation stage 11 for movingthe wafer 1 in a radial direction (one axis direction). The wafer stage103 allows the whole sample surface to be illuminated spirally byconducting rotation scanning and translation scanning. A stagecontroller 114 controls rotational speed and translation speed so that adesired area may be able to be illuminated. An imaging optical system 12and a CCD camera 13, which will be described later, are used todesignate the rotation center of the wafer and to monitor anillumination position and an angle that a longitudinal direction of anillumination and a translation direction of the stage make.

Although the explanation was given in the example where one illuminationoptical system and one detection optical system are provided in FIG. 1,it is all right to adopt a configuration where a plurality ofillumination optical systems and detection optical systems exist byproperly combining a slant illumination optical system of conductingillumination from a low elevation angle to the sample, a verticalillumination optical system of conducting illumination from anapproximately vertical direction to the sample, also a low-angledetection optical system of performing detection at a low elevationangle to the sample, and a high-angle detection optical system ofperforming detection at a higher elevation angle than that of thelow-angle detection optical system to the sample. Making an annotationon the slant illumination optical system and the vertical illuminationoptical system, since the use of the slant illumination optical systemcan improve detection sensitivity and the use of the verticalillumination optical system can improve a classification capability ofdefects, any of the two may be used properly according to a use.Moreover, it becomes possible to improve accuracy of defectclassification by using a combination of these illumination opticalsystems and/or detection optical systems. For example, for a convexdefect, when illumination is conducted from a slant direction, largescattered light can be detected with the low-angle detection opticalsystem; for a concave defect, when illumination is conducted from thevertical direction, large scattered light can be detected with thehigh-angle detection optical system. Moreover, if there exist aplurality of detection optical systems in different azimuth angledirections, as shown in FIG. 3, it poses no problem. That is, FIG. 3 isa diagram of one example embodiment of the defect inspection apparatusaccording to the present invention when viewed from a viewpoint aboveit, showing a wafer 1, the illumination optical system 101, anddetection optical systems 102 a to 102 f. The detection optical systems102 a to 102 f are constructed with image formation systems 8 a to 8 fand the photodiode arrays 9 a to 9 f, respectively. A detection signalundergoes necessary amplification, noise processing, andanalog-to-digital conversion in the analog circuit. In the signalprocessing section, an addition operation and the defect determinationare performed on a plurality of optical signals generated by scatteringfrom approximately the same area, which will display the defect map inthe map output section via the CPU (not shown). Here, explaining aconfiguration of the detection optical system, each of the imageformation systems 8 a to 8 f is constructed with a collective lens, animage intensifier, and an imaging lens (not shown).

In this way, if the detection optical systems existing in a plurality ofazimuth angles are used, in the case where angular characteristics ofthe scattered light rays occurring change due to the size and shape of adefect, a film kind of the sample, and surface roughness, the inspectioncan be performed by selecting a detection optical system that has smallnoise and is capable of detecting more of the scattered light rays fromthe defect; therefore, it becomes possible to improve the detectionsensitivity. Regarding arrangement of the detection optical systems,although an example where six detection optical systems are arranged indifferent azimuth angle directions is shown in FIG. 3, the number of thedetection optical systems does not necessarily need to be six and theazimuth angle directions in which they are arranged has no limitation,either. Moreover, the plurality of detection optical systems does notneed to be arranged approximately in the same elevation angle, and itposes no problem that they are arranged in differ elevation angles.Further, detectors do not need to be arranged approximately in the sameazimuth angle. In FIG. 3, although the laser illumination is conductedin a direction parallel to the longitudinal direction of theillumination, the longitudinal direction of the illumination and adirection in which the laser is irradiated do not need to beapproximately the same, and the illumination may be performed from adifferent direction. By conducting the illumination from differentdirections, the scattered light distribution occurring due to a defect,such as COP and micro scratch, can be changed; therefore, classificationperformance can be improved by a combination of the detection signals ofthe detectors existing in a plurality of azimuth directions.

Next, a method for performing inspection with high sensitivity byirradiating approximately the same area of the sample surface aplurality of times while suppressing damage to the sample will beexplained. The stage holding the sample is translating at anapproximately constant speed in the radial direction (R direction) whilemaking rotation. A distance that the stage makes in the radial directionat a time point of having made approximately one rotation is called afeed pitch. The whole sample surface is spirally scanned by performingrotation and translation. The present invention is characterized in thatillumination is conducted on approximately the same area a plurality oftimes by setting the length of an illumination field to be longer thanthe feed pitch length. The inspection method will be explained in detailbelow. First, the method for illuminating approximately the same area ofthe sample a plurality of times will be explained using FIG. 4. FIG. 4is an explanatory diagram of a case where the length of the illuminationfield 20 is four times a feed pitch 26, and the illumination isconducted to a defect 25 four times. If the first illumination isconducted to the defect 25 at time t1, the wafer makes approximately onerotation at time t2, the illumination field proceeds in the radialdirection by a length of the feed pitch 26, and the defect 25 isilluminated again. After this, at time t3 and at time t4, the wafermakes approximately one rotation and allows the defect 25 to beilluminated. That is, in the case of FIG. 4, the defect 25 can beilluminated four times and the detected light undergoes additionprocessing in the analog circuit or signal processing section. Byilluminating approximately the same area of the sample a plurality oftimes in this way, a minute defect existing on the sample surface can beinspected with high sensitivity without causing damage to the sample dueto temperature rise of an irradiated portion. Incidentally, it is notnecessary that the number of times of illumination is four times, andany times may be all right as long as the number expresses a pluralityof times of illumination. Moreover, regarding the illumination opticalsystem, not only a linear illumination may be generated using the beamexpander and the cylindrical lens, but also a long illumination fieldmay be generated by dividing the laser beam using a Wollaston prism andaligning the divided laser beams in the radial direction andillumination may be performed on the sample surface. Although aplurality of scattered light rays from approximately the same area areadded and the defect detection is performed, at this time, the pluralityof scattered light rays may be averaged, not being added, to perform thedefect detection. Moreover, a distance between two illumination fieldsthat were divided can be adjusted freely, and it poses no problem thatillumination is conducted with beams overlapped or with the beamsseparated. By this adjustment, it becomes possible to adjust the numberof times of illumination that will be done on approximately the samearea. Furthermore, not only the divided laser beams may be aligned toconduct illumination from approximately the same direction, but also,for example, both the slant illumination optical system and the verticalillumination optical system may be used simultaneously to conductillumination so that the two illumination fields may be arranged side byside to conduct illumination. By this simultaneous illumination, thesame defect will be illuminated approximately from vertical and slantdirections in a single inspection, which will lead to attainment ofimproved performance of defect classification by using a difference indetection elevation angle and in detection azimuth. Moreover, theillumination light does not need to be a linear illumination, and otherilluminations, such as a spot illumination and an area illumination, canbe used. If an illumination field length is longer than the pitchlength, approximately the same area can be irradiated a plurality oftimes. Similarly, a deformed spot illumination like an ellipticalillumination can irradiate approximately the same area a plurality oftimes provided that the illumination field length is longer than thefeed pitch length.

Next, a method for detecting a defect on the sample surface using thedefect inspection apparatus will be explained using a defect detectionprocessing flow of FIG. 5. First, by recipe setup, inspectionconditions, such as an illumination direction and sensor sensitivity,are set up (Step 120). Setup of the length of the illumination field,the feed pitch, and the processing method that will be done to thescattered light are also included in it. Then, wafer scanning is started(Step 121) and signal processing of the plurality of scattering lightrays being set up by the recipe setup is performed to detected scatteredlight (Step 122). Next, the defect determination is performed based onthe signal that was processed (Step 123), and a defect map is outputted(Step 124).

In the foregoing, the explanation about the apparatus and method forconducting irradiation on approximately the same area of the samplesurface was given. However, a plurality of scattered light rays inapproximately the same area may have errors of detection timings due toinclination of the illumination apparatus, the sensor, etc. Next, thiserror of the detection timing will be explained using FIG. 6A. A casewhere the defect 25 is illuminated three times in the illumination field20 is considered. From a first rotation round, a detection signal 30 ais obtained from the defect 25; from a second rotation round, adetection signal 30 b is obtained; and from a third rotation round, adetection signal 30 c is obtained, respectively. When the scatteredlight rays of the defect of the detection signals 30 a, 30 b, and 30 care detected at timing 31, it is possible to add them with excellentaccuracy like a detection signal 32; therefore, it is possible toimprove the detection sensitivity. However, when an angle 33(hereinafter, written as an angle shift) of the longitudinal directionof the illumination field and the translation direction of the stage bythe illumination apparatus and the sensor having inclinations is large,an error will arise between detection positions 38 a, 38 b correspondingto a detection angle that is a rotation angle of the sample controlledby the stage controller and irradiation positions 39 a, 39 b of thesample by the laser beam, which consequently will cause an error indetection timings of a plurality of scattered light rays by the samedefect, like detection signals 34 a, 34 b, and 34 c of FIGS. 6B and 6C.Therefore, since the scattered light from the same defect is notdetected within the timing 31, the detected light cannot be addedaccurately and improvement of the detection sensitivity becomesdifficult as a detection signal 35. That is, in order to add a pluralityof scattered light rays with excellent accuracy, it is necessary todetect the scattered light rays of the same defect approximately at thesame timing and to perform addition approximately at the same timing.For this purpose, a mechanism of monitoring and adjusting theillumination position, the angle shift, sensor inclination is needed andit is necessary to adjust timings 36, 37 of addition start of theplurality of detection signals. Below, an explanation will be givenabout a calibration method and an apparatus for calibrating theparameters in order to correct errors of detection timings that aplurality of scattered light rays have from approximately the same areain the present invention.

FIG. 7 shows a flow of calibration. A reference wafer having a referencepoint at which a defect such as PSL is formed, its position being known,is set in the stage (Step 130). The wafer is rotated while beingobserved with the image formation optical system 12 and the CCD camera13 in FIG. 1, and the rotation center of the stage is designated basedon an amount of movement of the defect position (reference point).Illumination is conducted in a direction that passes through therotation center and is approximately parallel to the translationdirection of the stage (Step 132). The illumination position and theangle shift are adjusted while the defect position is being observedwith the image formation optical system 12 and the CCD camera 13, and ageometrical relation between the illumination field and the detectionrange is adjusted by changing a sensor angle (Step 133). The same defectis measured using the reference wafer that was used at Step 130, and thedetection timing is monitored (Step 134). Step 133 and Step 134 arerepeated, and the timings at which the same defect is detected aplurality of times are made concordant with each other (Step 135). Anerror of the detection timing that cannot be corrected by a physicaladjustment mechanism is adjusted by the signal processing section (Step136).

Below, detailed explanation will be given for Step 130 to Step 136.Steps 130 and 131 will be explained using FIG. 8. FIG. 8 shows anexplanation about a method for designating the rotation center of thestage. The reference wafer on which PSL is applied near the wafer centeris set to the stage, and the stage is moved so that a central part ofthe wafer approximately comes in a view field with the CCD camera. Adefect that is processed by FIB or the like may be used, even if it isnot a PSL. It is desirable that the reference wafer is a wafer havinglarge surface roughness. This is because of a reason that will bedescribed later, namely, this is intended to monitor the illuminationposition by detecting the scattered light due to the surface roughness.If a defect enters in the view field, an image is tried to be acquiredwhile the stage is being rotated. A case where the defect is detected inthree images when the rotation causes positions of the defect to bechanged is considered. In a detection range 40 of the CCD, the defectsare detected at positions 25 a, 25 b, and 25 c. Since the stage isrotating, the positions 25 a, 25 b, and 25 c draw a circular arc 41. Ifan arbitrary pixel 43 on the CCD is set to an origin, coordinates of thepositions 25 a, 25 b, and 25 c are defined. If there are minimum threepoints, it is possible to compute rotation center coordinates 44 of thestage. When the curvature of the circular arch 41 is large, the viewfield is brought close to the rotation center and the above-mentionedprocedure is performed again with the PSL nearer the rotation center.Although the explanation was given this time for the case where thethree defects were detected, computation may be performed from moredefect positions. Such a procedure gives an effect of improvingcoordinate accuracy of the rotation center.

FIG. 9 will explain Step 132. FIG. 9 illustrates a method for conductingthe illumination in a direction that passes through the stage rotationcenter and is approximately parallel to the translation direction (Rdirection) of the stage. A case where a defect 25 d exists in thevicinity of the rotation center 44 is considered. When the stage istranslated, since the defect position (reference point) moves from thepoint 25 d to a point 25 e, it is possible to compute a translationdirection 45 of the stage from the coordinates. Illumination isconducted on a direction 46 that passes through the rotation center 44and is parallel to the translation direction 45.

Next, Step 133 will be explained. FIG. 10 is an explanation of themethod for monitoring the illumination position. Since a reference wafer47 is a wafer having large surface roughness, which was described in theexplanation of Steps 130, 131, the scattered light occurs much fromsurface unevenness. That is, the scattered light occurs from an areawhere illumination is conducted, and it is possible to designate theillumination position by monitoring the scattered light with the CCD 13.A method for correcting the angle shift will be explained using FIGS.11A and 11B. FIG. 11A shows portions of the laser beam 100, the beamexpander 3, and the collective lens 7 extracted from the illuminationoptical system. Just before entering into the expander, the laser beamhas a shape of an almost complete circle 50 when viewed from itstraveling direction. If after passing through the expander,magnification is changed in a direction 56 b, it becomes of a beam shapelike 51B. Then, the laser beam is reduced in the same direction as thedirection 56 b with the collective lens 7, and the illumination isconducted with the form 52 b on the wafer. The expander 13 is rotatablein a plane 55 that is perpendicular to a laser traveling direction,which enables a beam shape irradiated on the wafer to be adjusted. Sincea direction in which the magnification after passing through theexpander changes can be altered as enlarged directions 56 a, 56 c bychanging an angle of the expander, and thereby beam shapes like 51 a, 51c are obtained, the illumination shape on the wafer can be adjusted like52 a, 52 c. Since the laser traveling direction does not change by thisadjustment, the irradiation position doe not change and the angle shiftcan be corrected in a plane perpendicular to the optical axis. Themethod for correcting the irradiation position can perform adjustment bychanging tilt angles of the mirrors 5, 6 in FIG. 1.

Correction of the inclination shift of a sensor plane with respect tothe illumination position will be explained using FIGS. 12, 13. In thecase of using a sensor 9 m whose sensor light receiving part height 60 bis large as compared to a widthwise direction 60 a of an illuminationrange like FIG. 12, since the detection timing of the defect scatteredlight depends on the illumination position, if the angle shift iscorrected with the above-mentioned technique, it becomes possible tocorrect the detection timing of the defect scattered light. In the caseof using a sensor 9 n whose sensor light receiving part height 60 c issmall as compared to the widthwise direction 60 a of the illuminationrange, since the detection timing of the defect scattered light dependson the position of the sensor light receiving part, the detection timingis adjusted by adjusting an angle 62 with a fine movement rotation stageetc.

Step 134 and 135 will be explained by FIG. 14. FIG. 14 illustrates anexplanation of a method for actually performing the inspection using thereference wafer having a reference point that is a defect whose positionis known.

In FIG. 14, the horizontal axis denotes angle θ and the vertical axisdenotes the amount of detected light, and FIG. 14 shows a relation ofthe amount of detected light and the angle θ when the same defect isdetected a plurality of times. If the encoder pulses give a count of npulses for one rotation, the encoder pulses 1st to n-th are included inthe first rotation round, (n+1)-th to 2 n-th are included in the secondrotation round, (2n+1)-th to 3 n-th are included in the third rotationround, and 1st pulse, (n+1)-th pulse, and (2n+1)-th pulse are shown asangle θ=0 degree. Considered is a case where the same defect isinspected, a detection signal like a signal 65 is obtained in the firstrotation round, and a detection signal 66 in the second rotation roundand a detection signal 67 in the third rotation round are obtained. Apulse shift 68 exists between the detection timing of the first rotationround and the detection timing of the second rotation round of the samedefect, and a pulse shift 69 exists between the detection timing of thesecond rotation round and the detection timing of the third rotationround thereof. The timing shift is produced in response to the angleshift and the magnitude of inclination of the sensor, and the magnitudesof the shifts of the defect detection timings 68, 69 are almostcomparable. Since one pulse of the encoder is equivalent to an angle ofabout several seconds or so, Step 133 and Step 134 are repeated untilthe detection timing shift becomes about over ten pulses.

Step 136 will be explained using FIG. 15. In FIG. 15A, an additionmethod of the scattered light rays in a memory section will bedescribed. A case where a sensor 70 detects the same defect three timesis considered. Symbols R1 to R7 denote sensor light receiving parts,respectively. In the first rotation round, the detection signal of thelight receiving part R1 of the sensor 70 is stored in memory 71 for theamount of light corresponding to one rotation, in the second rotationround, the detection signal of the light receiving part R2 is stored inthe memory 71, and in the third rotation round, the detection signal ofthe light receiving part is stored in the memory 71. If the detectiontiming of the defect has a shift, a detection signal 75 is obtained inthe first rotation round, a detection signal 76 is obtained in thesecond rotation round, and a detection signal 77 is obtained in thethird rotation round, as shown in FIG. 15B. Therefore, it is difficultto perform addition with high accuracy. Even when the correction isperformed in the steps 134, 135, it is actually difficult to make thedetection signals agree with each other without an error as small as onepulse of the encoder due to mechanical precision, and consequently pulseshifts 78, 79 among the detection signals 75 to 77 exist. A case wherethe pulse shifts 78, 79 are comparable numbers of pulses that make ashift of k pulses is considered. If the detection signals of the firstrotation round are specified to be encoder pulse 1 to (n+k), and thedetection signals are stored in the memory 71, the storage start pulseof the second rotation round is specified as (n+1+k), and the storagestart pulse of the third rotation round is specified as (2n+1+2k), itbecomes possible to correct the detection signals so that the defectscattered light rays can be detected at approximately the same timing asshown in FIG. 15C, which goes with a detection signal 80 for the firstrotation round, a detection signal 81 for the second rotation round, anda detection signal 82 for the third rotation round. Therefore, itbecomes possible to add the scattered light signals of the same defectat approximately the same timing. Actually, there is a possibility thatthe pulse shifts 78, 79 have different number of pulses. In that case,what is necessary is just to find an average value together with thepulses 78, 79, and to designate the value as a k value and then performthe above-mentioned correction. Alternatively, it is all right to set amedian of a plurality of pulse shifts as the k value and perform thecorrection. In the case where the median is used, when the plurality ofpulse shifts include outliers, the k value can be obtained that is notaffected largely by the outliers. Alternatively, it may be all right toset a difference of two pulse shifts to the k value and perform thecorrection. When the plurality of pulse shifts are comparable values, avalue comparable to the k value computed by averaging can be obtained.Moreover, the correction of errors of detection timings that a pluralityof scattered light rays from approximately the same area have may beperformed using the k value being set up in advance using the referencewafer. In this case, it is not necessary to compute the k value for eachsample that undergoes the defect inspection, reduction of a throughputcan be prevented. Further, the k value may be computed sequentiallyusing a sample having a reference point that is a defect whose positionis known on the sample surface instead of the reference wafer, and thecorrection may be performed. In this case, since the detection timingcan be corrected after finding the k value for each sample, ahigher-precision addition result can be obtained.

Next, a method for switching memory that stores detection signals willbe explained. Although in the explanation in the above-mentioned FIG.15, memory storing the detection signals is switched for each one oflight receiving parts, the storage memory may be switched for each twoof the light receiving parts as shown in FIG. 16. In the first rotationround, the detection signals of the light receiving part R1 are storedin memory 72; in the second rotation round, the detection signals of thelight receiving part R3 are stored in the memory 72; and in the thirdrotation round, the detection signals of the light receiving part R5 arestored in the memory 72. Moreover, in the first rotation round, thedetection signals of the light receiving part R2 are stored in memory73; in the second rotation round, the detection signals of the lightreceiving part R4 are stored in the memory 73; and in the third rotationround, the detection signals of the light receiving part R6 are storedin the memory 73. Since by configuring the light receiving part asdescribed above, an area that can be inspected in a single rotationbecomes doubled, it becomes possible to double the feed pitch, andconsequently the throughput can be improved. Although the explanationwas given for the example where the storage memory is switched for everytwo light receiving parts, the number of switchover does not need to betwo times and it is possible to enlarge the feed pitch further.

Next, the inspection method in the wafer peripheral part will beexplained. In the conventional technology, scanning is performedspirally, and when the illumination field reaches an outermostperipheral part, the inspection will be completed. In the presentinvention, the method and the apparatus according to the presentinvention may be such that, when the illumination field reaches theoutermost peripheral part, the stage movement to the radial direction isstopped, rotational scanning is concentrically performed, and thedetection signal of the next one rotation is stored in the same memorywithout switching the storage memory of the detection signal. FIG. 17explains a method for changing the storage memory in the case where theillumination field end reaches the outermost peripheral part at a timewhen the second rotation round completed and the rotational scanning isbeing performed concentrically in the third rotation round. In the firstrotation round, the detection signal of the light receiving part R1 isstored in memory 74; in the second rotation round, the detection signalof the light receiving part R2 is stored in the memory 74; and in thethird rotation round, the detection signal of the light receiving partR2 is stored in the memory 74. This scheme makes it possible to increasethe number of additions in the outermost peripheral part. Here, sincethe concentric scanning is performed in an outer peripheral part, it ispossible to freely set up the number of additions. In an innerperipheral part, a relation between the illumination field length andthe feed pitch length decides the number of times of illumination beingable to illuminate approximately the same area. However, the number oftimes of illumination in the outer peripheral part does not need to beequal to the number of times of illumination in the inner peripheralpart, and it poses no problem even if that number is made larger thanthe number of times of illumination in the inner peripheral part.

Next, one example of the inspection method of the present invention willbe explained. In the conventional technology, since linear velocity isslow in the sample inner peripheral part and the linear velocity becomesfast in the sample outer peripheral part; in the outer peripheral part,an irradiation time of the laser beam becomes short, and the detectionsensitivity falls. In the present invention, it is possible to freelyalter the number of additions even during a one-time inspection. It ispossible to set up the number of additions arbitrarily by switching thefeed pitch and the storage memory, for example, in this way: the numberof additions is set to one time in an area from the rotation center to50 mm or less, the number of additions is set to two times in an areawithin 100 mm, and the number of additions is set to three times in anarea 101 mm or more. By this setup, it becomes possible to prevent thesensitivity from falling in the sample outer peripheral part, and tokeep the detection sensitivity constant on the whole sample surface.

In the foregoing, the invention made by the present inventors wasexplained concretely based on the embodiment. However, it is needless tosay that the present invention is not restricted to the above-mentionedembodiment but can be altered variously without departing from thespirit and scope of the present invention. Incidentally, although aplurality of scattered light rays from approximately the same area areadded to perform defect detection, at this time, the plurality ofscattered light rays may be averaged, not performing addition, to detectthe defect. Moreover, the defect inspection apparatus may be equippedwith a mechanism that enables the number of irradiation on approximatelythe same area to be set up freely by adjusting the length of theillumination field of the illumination, the translation speed of thestage, or the like according to a characteristic of an object underinspection etc. As described above, according to the embodiment of thepresent invention, the detection sensitivity can be improved byilluminating the same defect a plurality of times in a singleinspection, correcting the errors of the detection timings that aplurality of scattered light rays occurring by it have, and then addingthem. Moreover, the inspection can be performed without losing thethroughput by using the photodiode array having a plurality of pixels.According to the embodiment of the present invention, it is possible tocalibrate addition timings of a plurality of scattered light rays, andtherefore it becomes possible to obtain a high-accuracy addition result.Moreover, it becomes possible to freely set up the number of additionsaccording to an inspection area and inspection mode by using an additiontiming correction function.

According to the present invention, the defect inspection method and thedefect inspection apparatus that are capable of inspecting a minutedefect existing on the sample surface with high sensitivity whilesuppressing damage to the sample can be provided.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A method for inspecting a defect of a surface of a sample,comprising: an irradiation step of irradiating a laser beam on thesample surface a plurality of times so that at least part of anillumination field of the laser beam on the sample surface illuminates afirst area of the sample surface each of the plurality of times; adetection step of detecting a plurality of scattered light rays from thefirst area caused by the plurality of times of irradiation; a correctionstep of correcting errors of detection timings for the plurality ofscattered light rays detected in the detection step; a step of at leastone of adding and averaging the plurality of scattered light rayscorrected in the correction step; and a defect determination step ofdetermining a defect on the sample surface based on a calculation resultin accordance with the step of at least one of the adding and averaging.2. The defect inspection method according to claim 1, wherein in thecorrection step, the correction is performed using a detection timingerror value that is determined using a reference wafer having areference point which is a position of a known defect.
 3. The defectinspection method according to claim 1, wherein in the correction step,the correction is performed using a detection timing error value that isdetermined using a position of a known defect on the sample.
 4. Thedefect inspection method according to claim 1, wherein in the correctionstep, the errors of detection timing are based on errors between adetection position corresponding to a detection angle that is a rotationangle of the sample controlled by a stage controller and an irradiationposition on the sample surface by the laser beam.
 5. The defectinspection method according to claim 1, wherein in the irradiation step,the laser beam is irradiated so that the illumination field on thesample is a line.
 6. The defect inspection method according to claim 1,wherein in the irradiation step, a plurality of sub-laser beams whichare divided from the laser beam are aligned and are made to irradiatethe sample surface.
 7. The defect inspection method according to claim1, wherein the correction step includes a first correction step ofcorrecting errors of detection timings by using physical adjustment anda second correction step of correcting errors of detection timings byusing signal processing adjustment.
 8. The defect inspection methodaccording to claim 7, wherein the second correction step is performedusing pulse shift of detection signals based on the plurality ofscattered light rays.